Selective measuring system for level and/or phase differences with digital calibration

ABSTRACT

To measure the attenuation and/or the phase angle of an impedance pad, an alternating current is passed alternately through the test pad and over a path independent of this pad into a heterodyning stage to generate, during a testing cycle, a beatfrequency oscillation of an amplitude and phase varying with the propagation characteristics of the test pad. The beat-frequency oscillation is then transmitted through a calibrated variableimpedance network constituting a digital attenuator or phase shifter which is progressively adjustable until its output matches a given reference voltage or phase. This reference parameter may be established during a calibrating cycle when the input wave bypasses the test pad in a zero or other reference position of the variable-impedance network while a feedback loop from a comparison circuit adjusts the generator of the reference signal to a balancing position; in a modified system, a fixed reference parameter may be alternately compared with corresponding parameters of a test wave and a standard wave, with concurrent stepping of two counters and resetting of one counter measures the difference in the magnitudes of these corresponding parameters.

United States Patent [72] Inventors Herbert Bayer Reutlingen; Peterllarzer, Eningen; Gunther Hoffmann, Eningen; Bernd Zabel, Reutlingen,all of, Germany [2]] Appl. No. 737,544 [22] Filed June 17, 1968 [45]Patented June 8, 1971 [73] Assignee Wendel 8; Goltermann Reutlingen,Germany [32] Priority June 15, 1967 [33] Germany [31] W 44178 [54]SELECTIVE MEASURING SYSTEM FOR LEVEL AND/0R PHASE DIFFERENCES WITHDIGITAL CALIBRATION 18 Claims, 3 Drawing Figs.

[52] US. Cl 324/57, 324/83 [51] Int. Cl GOlr 27/00 [50] Field of Search324/57 D, 57 NBC, 83 D, 57 ABC, 57 A, 57 PF, 57, 83

[56] References Cited UNITED STATES PATENTS 2,622,127 12/1952 Alsberg eta1 324/57 3,355,662 11/1967 Haynie et al 3,243,699 3/1966 KummervABSTRACT: To measure the attenuation and/or the phase angle of animpedance pad, an alternating current is passed al temately through thetest pad and over a path independent of this pad into a heterodyningstage to generate, during a testing cycle, a beat-frequency oscillationof an amplitude and phase varying with the propagation characteristicsof the test pad. The beat-frequency oscillation is then transmittedthrough a calibrated variable-impedance network constituting a digitalattenuator or phase shifter which is progressively adjustable until itsoutput matches a given reference voltage or phase. This referenceparameter may be established during a calibrating cycle when the inputwave bypasses the test pad in a zero or other reference position of thevariable-impedance network while a feedback loop from a comparisoncircuit adjusts the generator of the reference signal to a balancingposition; in a modified system, a fixed reference parameter may bealternately compared with corresponding parameters of a test wave and astandard wave, with concurrent stepping of two counters and resetting ofone counter measures the difference in the magnitudes of thesecorresponding parameters.

as t may W l CONVERTER f I n ea 63 67 [we 79 qq $68K:

PATENTEDJUN 1ml 3584.295

SHEET 2 OF 3 Herberf Bayer Pefer Harzer GUnfher Hoffmann Bernd Zabel INVE NTORS.

Fig. 2

BY jams K Attorney SELECTIVE MEASURING SYSTEM FOR LEVEL AND/R PHASEDIFFERENCES WITI-I DIGITAL CALIBRATION Our present invention relates toa system for ascertaining the propagation characteristics of animpedance network, hereinafter referred to as a test pad, by determiningthe effect which this network has upon the amplitude and phase, oreither of these parameters, of an alternating current passedtherethrough.

The general object of our invention is to provide reliable andconveniently operable means for making this determination by passing aninput wave of stable frequency alternately through the test pad and overa path independent of that pad to a comparison circuit to determine thedifference in amplitude and/or phase of the respective outputoscillations.

ln somewhat more general terms, our invention aims at providing improvedcircuitry for comparing the amplitudes and/or phases of two oscillationsof like frequency occurring at different times.

In accordance with our present invention, we realize the afore'statedobjects by providing a variable-impedance network, operable as anattenuator or a phase shifter, which receives the oscillation to bemeasured and is progressively adjustable until its output matches areference voltage or phase; this variable-impedance network consists ofa plurality of calibrated sections each designed to introduce apredetermined incremental value of the parameter which the network isdesigned to modify. From the combination of sections effectivelyconnected in circuit when the difference between the network output andthe reference parameter is zero, an indicator electronically coupledwith the network registers the value of the introduced attenuation orphase shift with an accuracy determined by the smallest incrementalvalue for which these sections are designed.

Advantageously, these incremental values are relatively proportionedaccording to the antilogarithms of respective powers of 2, four suchbinary sections (eg of numerical weights 1, 2, 4, 8, or 1, 2, 2, 4 interms of actual or simulated damping effect) being then required toreproduce the numerical values from Oto 9. If conversion from a binaryto a decimal progression is desired, the network may thus include four nsections in cascade where n is the number of decades to be encompassed.

According to another feature of our invention, the stepping of thevariable-impedance network through its various digital combinationsproceeds under the control of a train of relative ly fast pulses as longas the absolute difference between the network output and the referenceparameter exceeds a predetermined magnitude, this rate beingsubstantially reduced upon said difference dropping below thatthreshold.

Since attenuation and especially phase are functions of frequency,another feature of our invention calls for the insertion of thecalibrated network into the output circuit of a heterodyning stage inwhich the input frequency, adjustable or spontaneously variable within acertain band, is mixed with a locally generally oscillation to producean AC signal of invariable beat frequency whose amplitude and phase varywith the corresponding parameters of the input wave. This beatfrequencysignal may also be used as a pacemaker for a pulse generator whichproduces the stepping pulses for the counter during either the fast orthe slow counting stage, preferably the latter.

The reference voltage or phase need not be a fixed value but may beestablished, periodically or otherwise, just before the testing ormeasuring cycle in which the desired parameter of the input wave is tobe ascertained. For this purpose, during a calibrating cycle immediatelypreceding the testing cycle, a wave of standard amplitude and/or phaseis passed along the transmission path including the calibrated networkwhich has been reset to zero or to some other predetermined position. Acomparison circuit, receiving the output of the calibrated networktogether with that of a generator producing the reference voltage orphase, then feeds back an imbalance signal to a control means for thisreference generator to adjust the latter until a balance is established.The reference generator is maintained by a clamping circuit in thisposition of adjustment so that the magnitude or phase of its outputremains substantially unchanged during the following testing cycle. Whenthe system is used to determine the attenuation and/or the phase shiftof a test pad, the standard wave is generally an input oscillationidentical with that used during testing but bypassing the test pad. Noclamping circuit is needed if, pursuant to another feature of ourinvention, two concurrently stepped counters are provided, one counterbeing reset before each calibrating (or testing) cycle so as to registeronly the difference between the two wave parameters as measured againsta common reference parameter.

The clamping of a reference voltage in a calibrating cycle may beaccomplished with the aid of a differential amplifier deriving a biasingpotential from a charged condenser as disclosed in commonly ownedapplication Ser. No. 693,431 filed Dec. 26, 1967 by Herbert Bayer. Forphase comparison, the generator of the reference signal may be avariable oscillator which, during a calibrating cycle, is locked in stepwith the standard wave by means of a voltage-responsive reactance (e.g.a varactor) whose control voltage, derived from the comparator output,is clamped during testing by a condenser, this in the same manner as thereference voltage used for amplitude measurement.

The invention will be more fully described with reference to theaccompanying drawing in which:

FlG. I is an overall circuit diagram of a testing system according tothe invention, having means for measuring both the amplitude and thephase of an input wave;

FIG. 2 is a more detailed diagram of certain constituents of the systemof H6. 1; and

FIG. 3 is a partial circuit diagram showing a modification of thesystem.

The system shown in FIG. 1 comprises a fixed-frequency oscillator 72whose output frequencyf is combined in a mixer 71 with an outputfrequency f, of a variable-frequency oscillator 39, thereby yielding atest frequency f passed by a narrowband filter 73 and an amplifier 74.The wave of frequency f is applied, in parallel, to input terminals 75and 76 of a test pad 32 and a standard pad 77 whose output terminals 33,34 are connected to respective bank contacts 35A, 35C of a switch 35controlled by a timer 69. The connection between terminal 34 and contact35C includes a manual switch 70 which may be reversed to apply thefrequency f directly to switch 35, without interposition of pad 77.Terminals 33, 34 and 75, 76 have been shown as plug-and-jack connectionsdesigned to facilitate the substitution of different networks for thepads 32 and 77.

Switch 35 is connected to the input of a broadband amplifier 38 by wayof a voltage divider 37, shown as an adjustable transformer, designed toselect a suitable amplitude level for the oscillations reaching thisamplifier. For this purpose, a detector circuit 80 integrates the outputof amplifier 38 and delivers a unipolar voltage to a level-controlcircuit including a condenser 83 in one input of a comparison circuit 82whose other input is energized from a source of reference voltage 81shown as a battery. Comparator 82 determines whether the chargingvoltage of condenser 83 falls within a predetermined range centered onthe reference voltage of battery 81; if this charging voltage is eithertoo low or too high, the output of the comparator reduces or increasesthe stepdown ratio of transformer 37 which is representative of anynetwork suitable for this purpose, e.g. a set of graduated impedancesalternately connectable in circuit with amplifier 38. Naturally, theoutput of amplitude gate 81 83 could also be used to adjust the gain ofthe amplifier in lieu of, or in addition to, changing its inputimpedance. The potential of condenser 83 may be read on an indicator 84.

Amplifier 38 works into a mixer 40 also receiving the local oscillationf from generator 39, the resulting beat frequency being the fixedfrequency f, produced by oscillator 72 and passed by a narrow-bandfilter 31 in the input ofa further amplifier 42. Thus, test frequency fmay be varied at will by adjustment of oscillator 39 without alteringthe frequency f of the beat oscillation appearing in the output ofamplifier 42. If such adjustability of the test frequency is notrequired, a fixedfrequency oscillator may be connected directly to theinput terminals 75 and 76, with omission of mixer 71 and oscillator 72.v

In accordance with our present invention, the oscillation f in theoutput of amplifier 42 is applied to the input ofa digital attenuator 43having a plurality of decadic stages; four such stages 430 (units), 43b(tenths), 43c (hundredths) and 43d (thousandths) have been illustratedby way of example. With the attenuator 43 calibrated in decibels, stages43a, 43b, 43c, 43d cover, respectively, the ranges of ll0 db., 0.1-ldb., 0.0l0.l db. and 0001-00] db.

Each, of these stages is controlled by a matrix of output leads fromcorresponding stages 46a, 46b, 46c, 46d of a reversible binary counter46 whose stages also control, via respective stages 47a, 47b, 47c, 47dofa coding network 47, a decimal indicator with stages 52a (0 9 db.) 52b(0.0 0.9 db.) 520 (0.00 0.09 db.) and 52d (0.000 0.009 db.) Network 47also works, through a decimal/analog converter 53, into an analogindicator 54 graduated in decibels. Owing to the provision of thisanalog indicator, the highest-order stage or stages of the digitalindicator 52 may be omitted, if desired without any loss of readingaccuracy. An output lead 85 may convey the attenuation reading to aprinter or some other recording device.

Counter 46 has two setting inputs 94, 94", originating at a switch 90,and a resetting input connected to an armature ofa switch 55 which isganged with the timer-controlled switch 35. Bank contacts 55A, 55C ofswitch 55 are unconnected whereas contact 558 is grounded.

Two pulse generators 48, 49 are designed to produce a fast train ofstepping pulses P, and a slow train of stepping pulses P,, respectively,these pulse trains being alternately fed to an input lead 94 of switch90 through an armature 50 of a relay 51. The winding of this relay isenergized by the output of a voltage comparator 44 receiving, on the onehand, the output of attenuator 43 by way of a detector 24 and, on theother hand, the output ofa differential amplifier 26 with tow inputsrespectively connected to a voltage source 22 and to a condenser 23. Aswitch 21, ganged with switches 35 and 55, is inoperative in itspositions 21A, 213 but in its third position 21C connects condenser 23through a low-pass filter 29 to an output lead 27 of comparator 44connected to ground via a large resistor lead 27 and amplifier 26 formpart of a feedback loop which, by the clamping effect of condenser 23,maintains the left-hand input of comparator 44 at a value exactlybalancing the voltage of detector 24 applied to its right-hand inputduring closure of switch 21. A further switch 28, also ganged withswitches 35 and 55, disconnects armature 50 from switch 90 in itspositions 288 and 28C, only its bank contact 28A being connected incircuit. This arrangement is generally similar to one disclosed in theaforementioned Bayer application Ser. No. 693,431.

Pulse generator 49 is shown to the output of amplifier 42, in parallelwith variable attenuator 43, in order to emit the stepping pulses P, ata cadence corresponding to the frequencyf or some harmonic orsubharmonic thereof.

The unrectified output of attenuator 43 is further trans mitted to theinput ofa digital phase shifter 56 which also may be subdivided intoseveral decadic stages, only two such stages 56 (for positive phaseangles +4 and 56" (for negative phase angles having been illustrated byway of example; stages 56' and 56" are controlled by correspondingstages 59 and 59" of a reversible counter 59 which work throughassociated stages 60, 60 of a coding network 60 into a digital indicator62 with stages 62 and 62" registering the logarithmic phase functions +3and -/3; another output of network 60 feeds an analog indicator 63through a decimal/analog converter 64. Again, a lead 79 indicates aconnection to a printer or other recording device.

Counter 59 is generally similar to counter 46 and has two setting inputs97, 97" emanating from a switch 98 whose feeder lead 97 includes aswitch 89, ganged with the timercontrolled switches 35,55, 21 and 28, abank contact contact 89A of this switch, and an armature 61" ofa relay61 having front and back contacts respectively connected to the outputsof pulse generators 48 and 49 via conductors and 96. The winding ofrelay 61 is controlled by the output of a phase comparator 57 receiving,on the one hand, a phase-reference signal from a variable oscillator 58and, on the other hand, the output of variable phase shifter 56.Oscillator 58 has a tank circuit including a varactor 65 whose controlvoltage is taken from a condenser 66 which is chargeable, by way ofalow-pass filter 67, from an output of comparator 57 via a switch 68 whenthe latter is on its bank contact 68C. Switch 68, whose bank contacts68A and 68B are unconnected, is also ganged with the aforementionedtimer-controlled, switches, as is a further switch 61 connected to aresetting input of counter 59; bank contacts 61A, 61C of the latterswitch are unconnected while a bank contact 61B thereof is grounded. Asin the case of the amplitude-measuring circuit including attenuator 43and voltage comparator 44, comparator 57 of the phase-measuring circuitalso has a feedback loop which includes switch 68, filter 67 andoscillator 58; upon closure of switch 68, which coincides with an openposition of switch 89, an output lead of the comparator (connected toground through a large resistor 99) charges the condenser 66 to avoltage which locks the oscillator 58 at an output frequency and phasematching those of the oscillation f in the output of phase shifter 56while the counter 59 is in its reset position. The reset positions ofcounters 46 and 59 do not necessarily correspond to zero damping ofattenuator 43 and zero phase shift or network 56, respectively, but maybe selected in any manner consistent with the transmissioncharacteristics of the circuit through which the standard wave f mustpass during calibration (including pad 77 if switch 70 is in itsillustrated position).

Since the two testing circuits 43, 46, 47, 52 and 56, 59, 60, and 62 areanalogous to one another, except for the facet that networks 43 and 56introduce attenuation and phase-shift increment, respectively, only thefirst of these circuits need to be be described in greater detail; also,inasmuch as all the stages of network 43 are of identical constructionand differ only in the magnitude of their (simulated) clamping factors,a description of one of these stages will suffice.

Thus, we have shown in FIG. 2 the units stage 43a of attenuator 43together with associated stages 46a, 47a and 52a of components 46, 47and 52. Stage 43a consists of four calibrated sections designated 143,243, 443 and 843, the hundreds digits of these reference numerals (l,"2, "4, "8") representing the logarithms of the simulated dampingfactors introduced thereby. Each of these attenuator stages isrepresented by a voltage-stepdown transformer whose stepdown ratiocorresponds to the apparent damping increment to be introduced thereby,i.e. of l db. in the case of transformer 143, 2 db. in the case oftransformer 243, 4 db. in the case of transformer 443 and 8 db. in thecase of transformer 843; thus, 206=log R where 8 is the dampingincrement in decibels and R is the turns ratio of the transformer. Withthese transformers connected in cascade, as shown, their stepdown ratiosare multiplicatively combined which, of course, corresponds to anadditive combination of their logarithmic damping increments.

As further shown for counter stage 46a, each of these stages consists offour flip-flops which have been designated 146, 246, 446 and 846, againin conformity with the respective damping increments. Each flip-flop hastwo output leads respectively controlling a pair of current gates whichhave been designated 101 and 102 in the case of transformer 143. Gate101, open in the set condition of flip-flop 146, connects the output ofthe secondary of transformer 143 across the primary of the succeedingtransformer 243; gate 102 opens when the flip-flop 146 is reset,bypassing the secondary of transformer 143 and directly connecting itsprimary across that of transformer 243. The several flip-flops areselectively tripped,

in a manner well known per se, under the control of stepping pulses Pfrom output lead 94' or 94" of switch 90, one or the other of theseleads being connected to input lead 94 if a positive or a negativeimbalance signal in the output of comparator 44 (FIG. 1) energizes therespective control lead 91 and 92 of the switch. With lead 91 energized,the sequence of operation of flip-flops 146, 246, 446 and 846 is suchthat the sum of the numerical weights (1, 2, 4, 8 of the associatedtransformers inserted in circuit varies progressively from 0 through 9upon the occurrence of every lOOOth stepping pulse. It will beunderstood that the switching occurs at every 100th pulse in stage 43b,every 10th pulse in stage 43c and every pulse in stage 43d.

Relay 51 operates when the voltage difference between the two inputs ofcomparator 44 is large, attracting its armature 50 and connecting thehighrate pulse generator 48 to lead 94 whereby counter 46 is r rapidlystepped in a forward or backward direction, depending upon the polarityof the voltage difference and the resulting energization of either lead91 or lead 92. When the absolute value of this voltage difference isreduced below a predetermined threshold, relay 51 releases so that thefinal adjustment of the attenuator 43 occurs at a relatively slow rate;this reduces or eliminates hunting of the counter.

In an analogous manner, relay 61 (FIG. 1) responds to the absolutemagnitude of the output of phase comparator 57 to connect input lead 97ofswitch 98, which controls the counter 59, to either ofthe two pulsesources 48, 49 via leads 95, 96.

Stage 47a (FIG. 2) of coding network 47 consists of a con ventionalbinary/decimal matrix which energizes any one of 10 indicator lamps 52ain response to the setting of a corresponding combination of theflip-flops of counter stage 4611. lndicators 52 and 54 display, on alinear scale, the sum of the logarithms ofthe attenuation incrementsintroduced by the binary sections 143 etc. of the several stages ofnetwork 43. By the same taken, the incremental phase-shifting sectionsof network 56 may operate in tandem of the antilogarithms of the valuesof angle function :[3 whose sum is linearly displayed by indicators 62and 63.

We shall now describe a complete operating period of the system shown inFIG. 1, including a calibrating cycle and a test cycle established bythe timer 69.

Let us assume that the system has just completed a test and thatswitches 21, 28, 35, 55, 61, 68 and 89 stand on their respective A"contacts. On being moved to their alternate C" positions by the timer69, these switches pass briefly through their intermediate or 13"positions in which the reset leads of counters 46 and 59 are grounded byswitches 55 and 61. This grounding switches all the flip-flops of eachcounter stage, or sets a predetermined combination of flip-flopscorresponding to a desired reference value for the attenuation and thephase shift, respectively. When the switches arrive on their C"contacts, input wave f passes through the heterodyning stage 38 42 andgives rise to beat-frequency oscillation f, which successively traversesthe attenuator 43 and the phase shifter 56. With the attenuator outputat its reference value, the closure ofthe feedback loops of comparator44 and 57 by switches 21 and 68 establishes a corresponding referenceparameter at the left-hand inputs of these comparators, this parameterbeing held substantially constant upon the subsequent switchover toposition A" in which the feedback loops are open and switches 90, 98 areconnected to pulse source 48 or 49 via relay armatures 50 and 61". Atthe same time, the input wave f, is forced to traverse the test pad 32whose transmission characteristics are reflected in a damping and aphase shift generally different from those of the standard wavetransmitted during the preceding calibrating cycle. If, for example,test pad 32 has a lower attenuation than standard pad 77, comparator 44delivers a positive" imbalance signal on lead 91 so that stepping pulsesP (FIG. 2) energize the forward" input 94' of counter 46, thus switchingthe flip flops thereof in rapid succession until the absence ofpotential on both leads 91 and 92 indicates that balance has beenrestored. Substantially at the same time, comparator 57 may energize theinput lead 97 or 97" so that phase shifter 56 is stepped forward orbackward until the imbalance signal disappears. Pulses P are, of course,representative of either pulse train P; or P the change from generator48 to generator 49 occurring not necessarily simultaneously in theattenuation and phase circuits.

When the attenuation counter 46 has come to rest, the test wave in theinput of phase shifter 56 will have the same amplitude as the standardwave during the preceding calibrating cycle; this eliminates theinfluence of amplitude variations upon the reading of the phase asdelivered by indicators 62 and 63.

Further timer-controllcd switches, not shown, may of course be providedto disconnect the indicators 52, 54, 62 and 63 from their associatedcoding networks 47 and 60 in the C" positions of switches 35 etc. so asto prevent fluctuation of the indicator outputs between the measuredtest value and the reference value.

The integrating network in the output of amplifier 38 should have alarge enough time constant to leave the setting of voltage divider 37unchanged between successive cycles (calibrating and testing); thepotential of condenser 83 will then correspond to the larger one of theamplitudes appearing in the input of the integrator during these twocycles. The upper and lower cutoff potentials established by comparator82 should, of course, be so chosen as to prevent overloading ofamplifier 38 while maintaining a satisfactory signal-to'noise ratio.With the voltage divider 37 thus traversed by the wave f during bothcalibrating and testing, its adjustment has no influence upon thereading of the indicators 52 and 54 which measure only the ratio of therespective damping factors of pads 42 and 77.

In the foregoing description it has been assumed that the attenuation ofstandard pad 77 is invariably greater than that of the pads 32 to betested. if a line of minimum attenuation is used as the standard (switch70 reversed), the stepdown transformers of FIG. 2 would have to bereplaced by stepup transformers. In the most general case, in which thedifferences in damping may be either positive or negative, indicator 52and networks 46, 47 would have to be divided into both positive andnegative stages (in the manner illustrated for the corresponding unitsof the phase-comparison circuit), with network 43 composed in part ofstepdown and in part of stepup transformers.

We shall now describe a modification ofthe system ofFlGS. l and 2 inwhich the difference between a certain parameter of a test wave and acorresponding parameter of a standard wave is registered upon successivecomparison thereof with a fixed reference parameter. While this featurewill be specifically discussed in connection with amplitudemeasurements, it will be understood that the phase-comparison circuitofFlG. 1 may be similarly modified.

In FIG. 3 the left-hand input of voltage comparator 44 is shownconnected to a source 45 of constant voltage. A second reversiblecounter 46', identical with counter 46, is connected in parallel withthe latter to the output of electronic switch so that both counters arestepped concurrently in response to the pulses on lead 94 in thepresence of an imbalance signal on conductor 91 or 92. Switch 55 isconnected to a resetting input of counter 46' whose output conductorslead to coding network 47 through armatures of a multilevel switch 20having bank contacts 20A, 20B and 20C. Switch 20, ganged with the othertimer-controlled switches, completes the connection to network 47 onlyin its calibrating position (contact 20C). Also, switch 28 of FIG. 1 hasbeen replaced by a switch 28 with bank contacts 28A, 28B and 28C, thisswitch connecting lead 94 to relay armature 50 in both testing" andcalibrating" positions.

The system of FIG. 3 operates as follows:

In testing position the amplitude of the wave to be measured is fedthrough attenuator 43 and detector 24, as before, to the right-handinput of voltage comparator 44 which generates an imbalance signal onlead 91 or 92 if this amplitude level exceeds or falls short of thereference voltage of battery 45. Counter 46 is stepped until the balanceis established; counter 46 (whose setting at this time will generallydiffer from that of counter 46) advances or retreats ineffectually inthe same rhythm, its output being open-circuited at switch 20. Next, inthe intermediate position B," counter 46 is reset to zero or some otherstarting position as signed to the pad 77; counter 46 does not move, itsinput circuit being open at switch 28. Finally, in the calibratingposition C, comparator 44 measures the reference voltage from battery 45against the amplitude of the standard wave as supplied by detector 24.If this amplitude exactly equals that measured during the precedingtesting cycle with the same setting of network 43, the comparator doesnot generate an imbalance signal and counter 46 remains at zero,indicating the absence of a level difference; if, however, such adifference does exist, both counters are stepped until the balance isrestored, the resulting setting of counter 46' being a measure of thatdifi'erence and being registered on indicators 52 and 53 (HO. 1).

If only the absolute values of the measured differences are of interest,counter 46' will be stepped only forward by pulses appearing on eitherlead 94 or lead 94".

The system of FIG. 3 will, of course, operate in essentially the sameway if calibration and testing occur in positions "A and C,"respectively, i.e. if pads 32 and 77 are interchanged.

With rapid alternation between calibrating and testing, the indicators52, 54, 62, 63 may have sufficient mechanical and/or electrical inertiato maintain their positions from one test cycle to the next.

Naturally, our disclosed system may also be utilized to compare theamplitudes (and/or, in the case of identical frequencies, the phases) ofdifferent oscillations propagated over the same transmission path,rather than a given oscillation traveling alternately over differentbranches (32, 77) of that path. lt will also be understood thatcalibrated resistors instead of transformers may be used in theattenuation stages of network 43, even though this is less advantageoussince it dissipates part of the available energy.

We claim:

1. A system for measuring electric waves, comprising:

circuit means for establishing a transmission path for an electric wavehaving a transmission -dependent parameter to be measured;

a network of variable transmission characteristics which affeet saidparameter of a wave propagated over said transmission path, said networkbeing connected to said circuits means and composed of a plurality ofcalibrated sections selectively insertable in said transmission pathsignal-generating means for establishing a reference parameter;

comparison means simultaneously connected to the outputs of saidsignal-generating means and of said network for matching said referenceparameter with said parameter of said wave to be measured;

selector means connected to be operated by said comparison means inresponse to the output thereof and coupled to said network forsuccessively inserting diiTerent combinations of said calibratedsections into said transmission path until said comparison meansindicates a balance;

a source of oscillations for applying a standard wave;

periodically operative switchover means for alternately applying saidstandard wave and said wave to be measured to the output of said circuitmeans for transmission over said path in a succession of calibrating andmeasuring cycles with establishment of said balance within each cycle;

and register means coupled to said selector means for translating anydifference between the settings of said selector means in successivecalibrating and measuring cycles into an indication of the desiredparameter.

2. A system as defined in claim 1 wherein the numerical weights of saidcalibrated sections in terms of their effect upon said desired parameterare the antilogarithms of respective powers of 2, said sections beingserially disposed for insertion in cascade.

3. A system as defined in claim 2 wherein said network is subdividedinto a plurality of decadic stages each composed of four calibratedsections.

4. A system as defined in claim 1 wherein said desired parameter isamplitude, said sections being calibrated in terms of attenuation.

5. A system as defined in claim 4 wherein said sections aretransformers.

6. A system as defined in claim 1 wherein said desired parameter isphase angle and said sections are calibrated in terms of phase shift.

7. A system for measuring electric waves, comprising:

means for generating a local oscillation;

circuit means for establishing a transmission path for an electric wavehaving at least one transmission-dependent parameter to be measured,said circuit means including a heterodyning stage for deriving a wave offixed beat frequency from said local oscillation and said electric wave;

a network of variable transmission characteristics which affect saidparameter in said wave of fixed beat frequency, said network beingconnected to said circuit means and composed of a plurality ofcalibrated sections selectively insertable in said transmission path ata location beyond said heterodyning stage;

signal-generating means for establishing a reference parameter;

comparison means simultaneously connected to the outputs of saidsignal-generating means and of said network for matching said referenceparameter with said parameter in said wave of fixed beat frequency;

selector means connected to be operated by said comparison means inresponse to the output thereof and coupled to said network forsuccessively inserting different combinations of said calibratedsections into said transmission path until said comparison meansindicates a balance;

a source of oscillations for applying a standard wave;

periodically operative switchover means for alternately applying saidstandard wave and said wave of fixed beat frequency to the input of saidcircuit means for transmission over said path in a succession ofcalibrating and measuring cycles with establishment of said balancewithin each cycle; and

register means coupled to said selector means for translating anydifference between the settings of said selector means in successivecalibrating and measuring cycles into an indication of the desiredparameter.

8. A system as defined in claim 7 wherein said selector means comprisesreversible binary counter means and a source of stepping pulses for saidcounter means, said source being coupled to the output of saidcomparison means for emitting said stepping pulses in response to animbalance signal.

9. A system as defined in claim 8 wherein said source is switchableprovided with switch means for changing from a relatively high to arelatively low pulse rate in response to a decrease of the absolutemagnitude of said imbalance signal below a predetermined threshold.

10. A system as defined in claim 9 wherein said source includes a pulsegenerator having input connections to said transmission path forenergization by said beat frequency.

11. A system as defined in claim 8, further comprising timer means forperiodically operating said switchover means to apply a wave to bemeasured and said standard wave to the input of said circuit means in afirst cycle and in a second cycle, respectively, said counter meansbeing provided with resetting means connected to said switchover meansfor establishing a starting position upon transition from one of saidcycles to the other.

12. A system as defined in claim 11 wherein said counter means comprisesa first counter and a second counter connected in parallel to saidsource for concurrent stepping thereby, said first counter beingoperative to insert said combinations of calibrated sections, saidsecond counter being connected to operate said register means and beingprovided with said resetting means.

13. A system as defined in claim 7, further comprising adjustablelevel-control means connected in said transmission path ahead of saidnetwork for alternate traversal by said standard wave and the wave to bemeasured.

14. A system as defined in claim 13 wherein said level-control meansincludes a broadband amplifier and an amplitude gate responsive to theoutput of said amplifier for holding the output amplitude thereof withinpredetermined limits.

15. A system as defined in claim 7, for measuring attenuation and phaseangle representing two transmission-dependent parameters, wherein saidnetwork comprises a first unit with sections calibrated in terms ofattenuation and a second unit with sections calibrated in terms of phaseshift, said units being connected in cascade to the output of saidheterodyning stage for receiving said wave of fixed beat frequencytherefrom.

16. A system for measuring electric waves, comprising:

circuit means for establishing a transmission path for an electric wavehaving at least one transmission-dependent parameter to be measured;

a network of variable transmission characteristics which affect saidparameter of a wave propagated over said transmission path said networkbeing connected to said circuit means and composed of a plurality ofcalibrated sections selectively insertable in said transmission path;

signal-generating means for establishing a reference parameter,including at least one variable voltage generator and control meanstherefor;

comparison means connected to the outputs of said voltage generator andof said network for matching said reference parameter with saidparameter of the wave to be measured, said comparison means beingprovided with a feedback loop extending from the output thereof to saidcontrol means;

selector means connected to be operated by said comparison means inresponse to the output thereof and coupled to said network forsuccessively inserting different combinations of said calibratedsections into said transmission path until said comparison meansindicates a balance;

a source of oscillations for supplying a standard wave;

periodically operative switchover means for alternately applying saidstandard wave and the wave to be measured to the input of said circuitmeans in a succession of calibrating and measuring cycles and forclosing said feedback loop during calibrating cycles only, therebyadjusting said voltage generator to establish a reference input for saidcomparison means matching the output of said network applied to saidcomparison means at the end of an immediately preceding measuring cycle;

reactive clamping means in said feedback loop for holding said referenceinput substantially constant during a subsequent measuring cycle; and

register means coupled to said selector means for translating thesetting thereof into an indication of the desired parameter.

17. A system as defined in claim 16 wherein said circuit means includesmeans for generating a local oscillation and a heterodyning stage forderiving a fixed beat frequency from said local oscillation and a waveto be measured and for feeding said beat frequency to said network.

18. A system as defined in claim 16 wherein said desired parameter isphase shift and said voltage generator comprises a variable oscillator,said control means including electronically variable reactance meansresponsive to the output of said comparison means for locking andoscillator in step with the output of said network.

1. A system for measuring electric waves, comprising: circuit means forestablishing a transmission path for an electric wave having atransmission-dependent parameter to be measured; a network of variabletransmission characteristics which affect said parameter of a wavepropagated over said transmission path, said network being connected tosaid circuits means and composed of a plurality of calibrated sectionsselectively insertable in said transmission path; signal-generatingmeans for establishing a reference parameter; comparison meanssimultaneously connected to the outputs of said signal-generating meansand of said network for matching said reference parameter with saidparameter of said wave to be measured; selector means connected to beoperated by said comparison means in response to the output thereof andcoupled to said network for successively inserting differentcombinations of said calibrated sections into said transmission pathuntil said comparison means indicates a balance; a source ofoscillations for applying a standard wave; periodically operativeswitchover means for alternately applying said standard wave and saidwave to be measured to the output of said circuit means for transmissionover said path in A succession of calibrating and measuring cycles withestablishment of said balance within each cycle; and register meanscoupled to said selector means for translating any difference betweenthe settings of said selector means in successive calibrating andmeasuring cycles into an indication of the desired parameter.
 2. Asystem as defined in claim 1 wherein the numerical weights of saidcalibrated sections in terms of their effect upon said desired parameterare the antilogarithms of respective powers of 2, said sections beingserially disposed for insertion in cascade.
 3. A system as defined inclaim 2 wherein said network is subdivided into a plurality of decadicstages each composed of four calibrated sections.
 4. A system as definedin claim 1 wherein said desired parameter is amplitude, said sectionsbeing calibrated in terms of attenuation.
 5. A system as defined inclaim 4 wherein said sections are transformers.
 6. A system as definedin claim 1 wherein said desired parameter is phase angle and saidsections are calibrated in terms of phase shift.
 7. A system formeasuring electric waves, comprising: means for generating a localoscillation; circuit means for establishing a transmission path for anelectric wave having at least one transmission-dependent parameter to bemeasured, said circuit means including a heterodyning stage for derivinga wave of fixed beat frequency from said local oscillation and saidelectric wave; a network of variable transmission characteristics whichaffect said parameter in said wave of fixed beat frequency, said networkbeing connected to said circuit means and composed of a plurality ofcalibrated sections selectively insertable in said transmission path ata location beyond said heterodyning stage; signal-generating means forestablishing a reference parameter; comparison means simultaneouslyconnected to the outputs of said signal-generating means and of saidnetwork for matching said reference parameter with said parameter insaid wave of fixed beat frequency; selector means connected to beoperated by said comparison means in response to the output thereof andcoupled to said network for successively inserting differentcombinations of said calibrated sections into said transmission pathuntil said comparison means indicates a balance; a source ofoscillations for applying a standard wave; periodically operativeswitchover means for alternately applying said standard wave and saidwave of fixed beat frequency to the input of said circuit means fortransmission over said path in a succession of calibrating and measuringcycles with establishment of said balance within each cycle; andregister means coupled to said selector means for translating anydifference between the settings of said selector means in successivecalibrating and measuring cycles into an indication of the desiredparameter.
 8. A system as defined in claim 7 wherein said selector meanscomprises reversible binary counter means and a source of steppingpulses for said counter means, said source being coupled to the outputof said comparison means for emitting said stepping pulses in responseto an imbalance signal.
 9. A system as defined in claim 8 wherein saidsource is switchable provided with switch means for changing from arelatively high to a relatively low pulse rate in response to a decreaseof the absolute magnitude of said imbalance signal below a predeterminedthreshold.
 10. A system as defined in claim 9 wherein said sourceincludes a pulse generator having input connections to said transmissionpath for energization by said beat frequency.
 11. A system as defined inclaim 8, further comprising timer means for periodically operating saidswitchover means to apply a wave to be measured and said standard waveto the input of said circuit means in a first cycle and in a secondcycle, respectively, said counter means being provided with resettingmeans connected to said sWitchover means for establishing a startingposition upon transition from one of said cycles to the other.
 12. Asystem as defined in claim 11 wherein said counter means comprises afirst counter and a second counter connected in parallel to said sourcefor concurrent stepping thereby, said first counter being operative toinsert said combinations of calibrated sections, said second counterbeing connected to operate said register means and being provided withsaid resetting means.
 13. A system as defined in claim 7, furthercomprising adjustable level-control means connected in said transmissionpath ahead of said network for alternate traversal by said standard waveand the wave to be measured.
 14. A system as defined in claim 13 whereinsaid level-control means includes a broadband amplifier and an amplitudegate responsive to the output of said amplifier for holding the outputamplitude thereof within predetermined limits.
 15. A system as definedin claim 7, for measuring attenuation and phase angle representing twotransmission-dependent parameters, wherein said network comprises afirst unit with sections calibrated in terms of attenuation and a secondunit with sections calibrated in terms of phase shift, said units beingconnected in cascade to the output of said heterodyning stage forreceiving said wave of fixed beat frequency therefrom.
 16. A system formeasuring electric waves, comprising: circuit means for establishing atransmission path for an electric wave having at least onetransmission-dependent parameter to be measured; a network of variabletransmission characteristics which affect said parameter of a wavepropagated over said transmission path said network being connected tosaid circuit means and composed of a plurality of calibrated sectionsselectively insertable in said transmission path; signal-generatingmeans for establishing a reference parameter, including at least onevariable voltage generator and control means therefor; comparison meansconnected to the outputs of said voltage generator and of said networkfor matching said reference parameter with said parameter of the wave tobe measured, said comparison means being provided with a feedback loopextending from the output thereof to said control means; selector meansconnected to be operated by said comparison means in response to theoutput thereof and coupled to said network for successively insertingdifferent combinations of said calibrated sections into saidtransmission path until said comparison means indicates a balance; asource of oscillations for supplying a standard wave; periodicallyoperative switchover means for alternately applying said standard waveand the wave to be measured to the input of said circuit means in asuccession of calibrating and measuring cycles and for closing saidfeedback loop during calibrating cycles only, thereby adjusting saidvoltage generator to establish a reference input for said comparisonmeans matching the output of said network applied to said comparisonmeans at the end of an immediately preceding measuring cycle; reactiveclamping means in said feedback loop for holding said reference inputsubstantially constant during a subsequent measuring cycle; and registermeans coupled to said selector means for translating the setting thereofinto an indication of the desired parameter.
 17. A system as defined inclaim 16 wherein said circuit means includes means for generating alocal oscillation and a heterodyning stage for deriving a fixed beatfrequency from said local oscillation and a wave to be measured and forfeeding said beat frequency to said network.
 18. A system as defined inclaim 16 wherein said desired parameter is phase shift and said voltagegenerator comprises a variable oscillator, said control means includingelectronically variable reactance means responsive to the output of saidcomparison means for locking and oscillator in step with the output ofsaid nEtwork.